linux-kernel - Re: [PATCH RFC] kvm: x86: add halt

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Message-ID: <54D3C296.7030207@redhat.com>
Date:	Thu, 05 Feb 2015 20:20:54 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
CC:	riel@...hat.com, rkrcmar@...hat.com, mtosatti@...hat.com
Subject: Re: [PATCH RFC] kvm: x86: add halt_poll module parameter



On 05/02/2015 19:55, Jan Kiszka wrote:
> > This patch introduces a new module parameter for the KVM module; when it
> > is present, KVM attempts a bit of polling on every HLT before scheduling
> > itself out via kvm_vcpu_block.
>
> Wouldn't it be better to tune this on a per-VM basis? Think of mixed
> workloads with some latency-sensitive and some standard VMs.

Yes, but:

1) this turned out to be very cheap, so a per-host tunable is not too bad;

2) it also affects only very few workloads (for example network
workloads can already do polling in the guest) so it only affects few
people;

3) long term anyway we want it to auto tune, which is better than tuning
it per-VM.

Paolo
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